Glossary

DARCYThe unit of measurement of rock permeability, i.e. the extent to which it will allow a fluid to flow through it.The permeability of most oil and gas reservoir rocks is measured in millidarcies, (thousandths of a Darcy).

DECLINE RATE The rate at which a well's production declines due to natural and sometimes man-introduced forces. Expressed in percent per year.

DELINEATION WELLA name for an appraisal well, usually one drilled specifically to determine the boundary of a discovered reservoir.

DEPLETION Refers to consumption of natural resources which are part of a company's assets. Producing oil, mining, gas and timber companies deal in products that cannot be replenished and, as such, are known as "wasting assets".

DEPTH MAPA relief map of a sub-surface geological structure where the contours relate to depths from the surface datum level. This is a further interpretation of a seismic time map.

DERRICKThe tower-like structure that houses most of the hoisting and lowering equipment & drilling controls.

DEVELOPMENT PHASEThe phase in which a proven oil or gas field is brought into production by drilling production (development) wells.

DEVELOPMENT WELLAny well drilled in the course of extraction of reservoir hydrocarbons.

DEVIATED WELL/HOLEA well whose path has been deliberately diverted from the vertical. Although relatively costly to drill, they are used particularly offshore to reach distant parts of a reservoir from a single platform. Deviated, or directional drilling up to 60º to 70º from the vertical is now fairly common. Greater deviation is possible with special equipment.

DIRECTIONAL DRILLINGThe technique of drilling at an angle from the vertical by deflecting the drill bit. Directional wells are drilled to develop an offshore lease from one drilling platform; to reach a pay zone where drilling cannot be done, such as beneath a shipping lane.

DISCOVERY WELL Exploratory well which discovers a new oil/gas field (see WILDCAT).

DIFFERENTIAL PRESSUREThe difference between the pressure in a well due to the mud column and the pressure in the surrounding rock at any point. See also Sticking.

DIP (DIPMETER)The inclination from the horizontal of to the surface of a geological structure. A Dip Meter indicates dip relative to a well bore.

DIRECTIONAL DRILLINGControlled drilling at a specified angle from the vertical.

DOWN DIPAn area of a structure where the top of the formation is lower than the point under consideration.

DOWN HOLEDown a well. The expression covers any equipment, measurement, etc., in a well or designed for use in one.

DOWNTIMEA period when any equipment is unserviceable or out of operation for maintenance etc.

DRAWWORKS Hoisting mechanism on a drilling rig which spools off or takes in the drilling line and thus raises or lowers the drill string and bit.

DRILL BIT The component at the end of the drill string that cuts the rock and makes a hole.

DRILL COLLAR Heavy-walled sections of pipe included at the bottom of the drill string to apply weight to the drill bit during drilling.

DRILL CUTTINGS Chips and small fragments of drilled rock that are brought to the surface by the flow of the drilling mud as it is circulated.

DRILL PIPE Steel pipe, in approximately 30-foot (9-meter) lengths, screwed together to form a continuous pipe extending from the drilling rig to the drilling bit at the bottom of the hole. Rotation of the drill pipe and bit causes the bit to bore through the rock.

DRILL STEM TESTS (DST)Conventional method of testing a formation to determine its potential productivity before installing production casing in a well. A testing tool is attached to the bottom of the drill pipe and placed opposite the formation to be tested which has been isolated by placing packers above and below the formation. Fluids in the formation are allowed to flow up through the drill pipe by establishing an open connection between the formation and the surface.

DRILL STRING String of individual joints of pipe that extends from the bit to the kelly and carries the mud down to, and rotates, the bit.

DRILLING FLUIDSWhile a mixture of clay and water is the most common drilling fluid, wells can also be drilled with air, nitrogen, natural gas, oil, or plain water as the drilling fluid.

DRY HOLE Generally refers to any well that does not produce oil or gas in commercial quantities.

DUAL COMPLETION Completion of a well in which two separate formations may be produced at the same time. Production from each zone is segregated by running two tubing strings with packers, or running one tubing string with a packer and producing the other zone through the annulus.

DRY GAS Natural gas composed mainly of methane with only minor amounts of ethane, propane and butane and little or no heavier hydrocarbons in the gasoline range.

E&P Abbreviation for exploration and production.

ENHANCED OIL RECOVERY (EOR)A process whereby oil is recovered other than by the natural pressure in a reservoir.

EXPLORATION/EXPLORATION WELLExploration is the process of identifying a prospective hydrocarbon region and structure, mainly by reference to regional, and specific, geochemical, geological and geophysical (seismic) surveys. An Exploration Well is a well drilled to test a potential but unproven hydrocarbon trap or structure where good reservoir rock and a seal or closure combine with a potential source of hydrocarbons.

FACIESIn geology, the composition and characteristics of a rock formation.

FARM INWhen a company acquires an interest in a block by taking over all or part of the financial commitment for drilling an exploration well.

FARMOUT An arrangement under which a portion of an interest in petroleum and natural gas rights is assigned in consideration for the assignee agreeing to explore or drill (and perhaps equip) one well or several wells at his sole expense; subsequent development and equipment cost, if any, and income and operating expenses are shaped by participants on an agreed basis.

FAULT/FAULT BLOCKA discontinuity in a rock formation caused by fracturing of the earth's crust. In oilfield terms a Fault Block is a compartment of a rock formation surrounded or partly surrounded by faults, which may have sealed in hydrocarbons separately from the rest of the formation.

FIELDAn area consisting of a single reservoir or multiple reservoirs all grouped on, or related to, the same individual geological structural feature or stratigraphic condition. The field name refers to the surface area, although it may refer to both the surface and the underground productive formations.

FILTER CAKE/FILTRATEBuild up of mud solids or filtrate on the wall of a well. This helps seal and stabilize the rock face, but too much can cause sticking of the drill string. See also Differential Pressure.

FISH/FISHINGAny unwanted object down a well, commonly the lower end of a drill string which has broken off. "Fishing" is trying to recover the Fish, using various attachments to the drill stem or wireline, known as fishing tools.

FLOWING PRESSURE Pressure registered at the wellhead of a flowing well.